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      1 //===- ObjCARC.h - ObjC ARC Optimization --------------*- C++ -*-----------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 /// \file
     10 /// This file defines common definitions/declarations used by the ObjC ARC
     11 /// Optimizer. ARC stands for Automatic Reference Counting and is a system for
     12 /// managing reference counts for objects in Objective C.
     13 ///
     14 /// WARNING: This file knows about certain library functions. It recognizes them
     15 /// by name, and hardwires knowledge of their semantics.
     16 ///
     17 /// WARNING: This file knows about how certain Objective-C library functions are
     18 /// used. Naive LLVM IR transformations which would otherwise be
     19 /// behavior-preserving may break these assumptions.
     20 ///
     21 //===----------------------------------------------------------------------===//
     22 
     23 #ifndef LLVM_LIB_TRANSFORMS_OBJCARC_OBJCARC_H
     24 #define LLVM_LIB_TRANSFORMS_OBJCARC_OBJCARC_H
     25 
     26 #include "llvm/ADT/StringSwitch.h"
     27 #include "llvm/ADT/Optional.h"
     28 #include "llvm/Analysis/AliasAnalysis.h"
     29 #include "llvm/Analysis/Passes.h"
     30 #include "llvm/Analysis/ValueTracking.h"
     31 #include "llvm/IR/CallSite.h"
     32 #include "llvm/IR/InstIterator.h"
     33 #include "llvm/IR/Module.h"
     34 #include "llvm/Pass.h"
     35 #include "llvm/Transforms/ObjCARC.h"
     36 #include "llvm/Transforms/Utils/Local.h"
     37 #include "ARCInstKind.h"
     38 
     39 namespace llvm {
     40 class raw_ostream;
     41 }
     42 
     43 namespace llvm {
     44 namespace objcarc {
     45 
     46 /// \brief A handy option to enable/disable all ARC Optimizations.
     47 extern bool EnableARCOpts;
     48 
     49 /// \brief Test if the given module looks interesting to run ARC optimization
     50 /// on.
     51 static inline bool ModuleHasARC(const Module &M) {
     52   return
     53     M.getNamedValue("objc_retain") ||
     54     M.getNamedValue("objc_release") ||
     55     M.getNamedValue("objc_autorelease") ||
     56     M.getNamedValue("objc_retainAutoreleasedReturnValue") ||
     57     M.getNamedValue("objc_retainBlock") ||
     58     M.getNamedValue("objc_autoreleaseReturnValue") ||
     59     M.getNamedValue("objc_autoreleasePoolPush") ||
     60     M.getNamedValue("objc_loadWeakRetained") ||
     61     M.getNamedValue("objc_loadWeak") ||
     62     M.getNamedValue("objc_destroyWeak") ||
     63     M.getNamedValue("objc_storeWeak") ||
     64     M.getNamedValue("objc_initWeak") ||
     65     M.getNamedValue("objc_moveWeak") ||
     66     M.getNamedValue("objc_copyWeak") ||
     67     M.getNamedValue("objc_retainedObject") ||
     68     M.getNamedValue("objc_unretainedObject") ||
     69     M.getNamedValue("objc_unretainedPointer") ||
     70     M.getNamedValue("clang.arc.use");
     71 }
     72 
     73 /// \brief This is a wrapper around getUnderlyingObject which also knows how to
     74 /// look through objc_retain and objc_autorelease calls, which we know to return
     75 /// their argument verbatim.
     76 static inline const Value *GetUnderlyingObjCPtr(const Value *V,
     77                                                 const DataLayout &DL) {
     78   for (;;) {
     79     V = GetUnderlyingObject(V, DL);
     80     if (!IsForwarding(GetBasicARCInstKind(V)))
     81       break;
     82     V = cast<CallInst>(V)->getArgOperand(0);
     83   }
     84 
     85   return V;
     86 }
     87 
     88 /// The RCIdentity root of a value \p V is a dominating value U for which
     89 /// retaining or releasing U is equivalent to retaining or releasing V. In other
     90 /// words, ARC operations on \p V are equivalent to ARC operations on \p U.
     91 ///
     92 /// We use this in the ARC optimizer to make it easier to match up ARC
     93 /// operations by always mapping ARC operations to RCIdentityRoots instead of
     94 /// pointers themselves.
     95 ///
     96 /// The two ways that we see RCIdentical values in ObjC are via:
     97 ///
     98 ///   1. PointerCasts
     99 ///   2. Forwarding Calls that return their argument verbatim.
    100 ///
    101 /// Thus this function strips off pointer casts and forwarding calls. *NOTE*
    102 /// This implies that two RCIdentical values must alias.
    103 static inline const Value *GetRCIdentityRoot(const Value *V) {
    104   for (;;) {
    105     V = V->stripPointerCasts();
    106     if (!IsForwarding(GetBasicARCInstKind(V)))
    107       break;
    108     V = cast<CallInst>(V)->getArgOperand(0);
    109   }
    110   return V;
    111 }
    112 
    113 /// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just
    114 /// casts away the const of the result. For documentation about what an
    115 /// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that
    116 /// function.
    117 static inline Value *GetRCIdentityRoot(Value *V) {
    118   return const_cast<Value *>(GetRCIdentityRoot((const Value *)V));
    119 }
    120 
    121 /// \brief Assuming the given instruction is one of the special calls such as
    122 /// objc_retain or objc_release, return the RCIdentity root of the argument of
    123 /// the call.
    124 static inline Value *GetArgRCIdentityRoot(Value *Inst) {
    125   return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0));
    126 }
    127 
    128 static inline bool IsNullOrUndef(const Value *V) {
    129   return isa<ConstantPointerNull>(V) || isa<UndefValue>(V);
    130 }
    131 
    132 static inline bool IsNoopInstruction(const Instruction *I) {
    133   return isa<BitCastInst>(I) ||
    134     (isa<GetElementPtrInst>(I) &&
    135      cast<GetElementPtrInst>(I)->hasAllZeroIndices());
    136 }
    137 
    138 
    139 /// \brief Erase the given instruction.
    140 ///
    141 /// Many ObjC calls return their argument verbatim,
    142 /// so if it's such a call and the return value has users, replace them with the
    143 /// argument value.
    144 ///
    145 static inline void EraseInstruction(Instruction *CI) {
    146   Value *OldArg = cast<CallInst>(CI)->getArgOperand(0);
    147 
    148   bool Unused = CI->use_empty();
    149 
    150   if (!Unused) {
    151     // Replace the return value with the argument.
    152     assert((IsForwarding(GetBasicARCInstKind(CI)) ||
    153             (IsNoopOnNull(GetBasicARCInstKind(CI)) &&
    154              isa<ConstantPointerNull>(OldArg))) &&
    155            "Can't delete non-forwarding instruction with users!");
    156     CI->replaceAllUsesWith(OldArg);
    157   }
    158 
    159   CI->eraseFromParent();
    160 
    161   if (Unused)
    162     RecursivelyDeleteTriviallyDeadInstructions(OldArg);
    163 }
    164 
    165 /// \brief Test whether the given value is possible a retainable object pointer.
    166 static inline bool IsPotentialRetainableObjPtr(const Value *Op) {
    167   // Pointers to static or stack storage are not valid retainable object
    168   // pointers.
    169   if (isa<Constant>(Op) || isa<AllocaInst>(Op))
    170     return false;
    171   // Special arguments can not be a valid retainable object pointer.
    172   if (const Argument *Arg = dyn_cast<Argument>(Op))
    173     if (Arg->hasByValAttr() ||
    174         Arg->hasInAllocaAttr() ||
    175         Arg->hasNestAttr() ||
    176         Arg->hasStructRetAttr())
    177       return false;
    178   // Only consider values with pointer types.
    179   //
    180   // It seemes intuitive to exclude function pointer types as well, since
    181   // functions are never retainable object pointers, however clang occasionally
    182   // bitcasts retainable object pointers to function-pointer type temporarily.
    183   PointerType *Ty = dyn_cast<PointerType>(Op->getType());
    184   if (!Ty)
    185     return false;
    186   // Conservatively assume anything else is a potential retainable object
    187   // pointer.
    188   return true;
    189 }
    190 
    191 static inline bool IsPotentialRetainableObjPtr(const Value *Op,
    192                                                AliasAnalysis &AA) {
    193   // First make the rudimentary check.
    194   if (!IsPotentialRetainableObjPtr(Op))
    195     return false;
    196 
    197   // Objects in constant memory are not reference-counted.
    198   if (AA.pointsToConstantMemory(Op))
    199     return false;
    200 
    201   // Pointers in constant memory are not pointing to reference-counted objects.
    202   if (const LoadInst *LI = dyn_cast<LoadInst>(Op))
    203     if (AA.pointsToConstantMemory(LI->getPointerOperand()))
    204       return false;
    205 
    206   // Otherwise assume the worst.
    207   return true;
    208 }
    209 
    210 /// \brief Helper for GetARCInstKind. Determines what kind of construct CS
    211 /// is.
    212 static inline ARCInstKind GetCallSiteClass(ImmutableCallSite CS) {
    213   for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
    214        I != E; ++I)
    215     if (IsPotentialRetainableObjPtr(*I))
    216       return CS.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser;
    217 
    218   return CS.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call;
    219 }
    220 
    221 /// \brief Return true if this value refers to a distinct and identifiable
    222 /// object.
    223 ///
    224 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses
    225 /// special knowledge of ObjC conventions.
    226 static inline bool IsObjCIdentifiedObject(const Value *V) {
    227   // Assume that call results and arguments have their own "provenance".
    228   // Constants (including GlobalVariables) and Allocas are never
    229   // reference-counted.
    230   if (isa<CallInst>(V) || isa<InvokeInst>(V) ||
    231       isa<Argument>(V) || isa<Constant>(V) ||
    232       isa<AllocaInst>(V))
    233     return true;
    234 
    235   if (const LoadInst *LI = dyn_cast<LoadInst>(V)) {
    236     const Value *Pointer =
    237       GetRCIdentityRoot(LI->getPointerOperand());
    238     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) {
    239       // A constant pointer can't be pointing to an object on the heap. It may
    240       // be reference-counted, but it won't be deleted.
    241       if (GV->isConstant())
    242         return true;
    243       StringRef Name = GV->getName();
    244       // These special variables are known to hold values which are not
    245       // reference-counted pointers.
    246       if (Name.startswith("\01l_objc_msgSend_fixup_"))
    247         return true;
    248 
    249       StringRef Section = GV->getSection();
    250       if (Section.find("__message_refs") != StringRef::npos ||
    251           Section.find("__objc_classrefs") != StringRef::npos ||
    252           Section.find("__objc_superrefs") != StringRef::npos ||
    253           Section.find("__objc_methname") != StringRef::npos ||
    254           Section.find("__cstring") != StringRef::npos)
    255         return true;
    256     }
    257   }
    258 
    259   return false;
    260 }
    261 
    262 enum class ARCMDKindID {
    263   ImpreciseRelease,
    264   CopyOnEscape,
    265   NoObjCARCExceptions,
    266 };
    267 
    268 /// A cache of MDKinds used by various ARC optimizations.
    269 class ARCMDKindCache {
    270   Module *M;
    271 
    272   /// The Metadata Kind for clang.imprecise_release metadata.
    273   llvm::Optional<unsigned> ImpreciseReleaseMDKind;
    274 
    275   /// The Metadata Kind for clang.arc.copy_on_escape metadata.
    276   llvm::Optional<unsigned> CopyOnEscapeMDKind;
    277 
    278   /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata.
    279   llvm::Optional<unsigned> NoObjCARCExceptionsMDKind;
    280 
    281 public:
    282   void init(Module *Mod) {
    283     M = Mod;
    284     ImpreciseReleaseMDKind = NoneType::None;
    285     CopyOnEscapeMDKind = NoneType::None;
    286     NoObjCARCExceptionsMDKind = NoneType::None;
    287   }
    288 
    289   unsigned get(ARCMDKindID ID) {
    290     switch (ID) {
    291     case ARCMDKindID::ImpreciseRelease:
    292       if (!ImpreciseReleaseMDKind)
    293         ImpreciseReleaseMDKind =
    294             M->getContext().getMDKindID("clang.imprecise_release");
    295       return *ImpreciseReleaseMDKind;
    296     case ARCMDKindID::CopyOnEscape:
    297       if (!CopyOnEscapeMDKind)
    298         CopyOnEscapeMDKind =
    299             M->getContext().getMDKindID("clang.arc.copy_on_escape");
    300       return *CopyOnEscapeMDKind;
    301     case ARCMDKindID::NoObjCARCExceptions:
    302       if (!NoObjCARCExceptionsMDKind)
    303         NoObjCARCExceptionsMDKind =
    304             M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions");
    305       return *NoObjCARCExceptionsMDKind;
    306     }
    307     llvm_unreachable("Covered switch isn't covered?!");
    308   }
    309 };
    310 
    311 } // end namespace objcarc
    312 } // end namespace llvm
    313 
    314 #endif
    315